Different types of films and similar constructions are known which are intended to be used to protect writing, especially data on documents, from attempts at tampering or to protect labels, such as price tags, from manipulation such as removing and readhering.
A known technology for protection of documents, such as identification cards and driver's licenses, consists of a multilayer film where microspheres are imbedded in a binder in one layer. Directly under the microspheres is a retroreflective customer-specific printing which can be observed under retroreflective conditions. Under this binder layer is a layer of pressure sensitive adhesive or heal-activated adhesive that serves to attach the film to the document. This type of construction, known, for example, from U.S. Pat. No. 3,801,183, U.S. Pat. No. 4,630,891, U.S. Pat. No. 5,080,463 and U.S. Pat. No. 5,169,707, offers a high degree of protection against tampering because the technology used in the retroreflective customer-specific printing cannot be imitated with traditional equipment. These constructions have the disadvantage, however, in that in order to view the customer-specific printing, equipment for retroreflective viewing is necessary.
DE-C-25 11 367 describes a tamper-proof recording medium comprising several layers where the document is laminated between two films, and where a pattern of adhesion-reducing coating has been applied to either the film or the document, so that very little or no adhesive bond is formed between the film and the document in areas where the adhesion-reducing coating is present. Normally, an attempt to tamper with a construction of this type would result in destruction of the document. In this case as well, however, professional tamperers are capable of removing the film carefully so that the substrate is not damaged.
The 3M Company brochure "3M ID Security Laminate Series 3000" Order No. 75-0500-00377-3 (24.5) IR (1992) from Minnesota Mining and Manufacturing Company describes a security laminate for use when only a certain amount of security is required. The laminate consists of two layers, between which lies a customer-specific printing which cannot be photocopied and which is destroyed when delamination is attempted.
Multilayer films for increasing the tamper-resistance of documents, where the tamper-resistance depends on the integrity of an intermediate layer having certain optical properties such as a kinegram or a hologram, are described in EP-A-253 089 and EP-A-401 466. The layer with optical properties is used in combination with a usual film, which itself has only a limited level of tamper-resistance. During an attempt at tampering, the hologram would be destroyed. The film in the area of the hologram is not transparent, however, and its extension over the entire document surface is, aside from the expense, not desirable because of the lack of transparency.
U.S. Pat. No. 4,876,123 and U.S. Pat. No. 5,082,702 describe tamper-proof films and tapes which are not transparent. The multilayer constructions are destroyed during attempts at tampering, both sides of the separated film layers showing a color printing which was originally concealed. A laminate which works in a similar way is described in U.S. Pat. No. 4,429,015 where fibers are pulled off of the document when attempts are made to remove the cover film and where the parts cannot be laminated together again without visible damage.
DE-A-28 03 434 describes a self-adhesive clear film having a layer of printed adhesive. When the document is tampered with, this printing remains on the document substrate. When a tamperer desires to exchange a photograph, for example, then the printed pattern must be reproduced on the new photo, a task which is relatively simple for a professional tamperer.
FR-A-2 451 081 describes a multilayer safety laminate for documents which contains an intermediate layer containing ink-filled microcapsules. Upon tampering, the capsules break and lead to discoloration of the document. This construction has the disadvantage, however, that the microcapsules reduce the transparency of the film and that normal use of the document can lead to breakage of the microcapsules.
DE-A-36 08 907 describes an adherable material for labels where an underlying tearable layer is destroyed when a surface layer is manually removed. This material is non-transparent, as the easily tearable layer is preferably an aluminum vapor coating which has adhesive on both sides, the adhesives differing in their adhesive strength. Between the tearable layer and the lower adhesive layer is a pattern of an adhesion-reducing coating. A cover layer can be printed as well and the entire construction used as a tamper-proof tape or safety label. Removal of the tape or label leads to an irreversible destruction of the largely two-dimensional nontransparent metal layer. Similar multilayer constructions with largely flat, very thin, non-transparent frangible layers are known from U.S. Pat. No. 3,925,584 and EP-A-170 712.
Further descriptions of tamper-proof labels can be found in U.S. Pat. No. 4,121,003 and U.S. Pat. No. 4,184,701. These labels have a transparent cover layer which has a printed pattern on its inner surface. The printed pattern is destroyed when the cover layer is removed.
U.S. Pat. No. 4,721,638 describes a non-transparent sticking material for preventing resticking, having an adhesive layer, a second layer laminated on one mail surface of the adhesive layer, a peeling agent layer laminated on the second layer and a first layer laminated on the peeling agent layer. The second and first layers are adhered temporarily via the peeling agent layer. The first layer once separated from the second layer cannot be restuck. The non-transparent sticking material is designed to avoid the use of polyester film and metallization in the construction.
A method of manufacturing tamper-proof documents which is related to one of the previously described multilayer films is described in DE-C-29 52 322. In this process, a document is laminated with a polyethylene film which has an adhesive coating comprising a radiation cross-linkable mass of ethylenically unsaturated oligomers or polymers based on urethane-modified polyepoxides. The film is laminated to the document in such a way that the crosslinkable mass comes in contact with the document. The laminate is then exposed to a crosslinking radiation (UV radiation). In this case, the document is provided with a very high level of protection from tampering. However, treatment of the laminate with UV radiation is necessary, which in practice would require undesirable investment in equipment and associated safety measures because every distribution point which issues such documents would need such UV-radiation equipment.
In summary, none of the numerous solutions to the problem provided by the state-of-the-art, described above, provides a satisfactory multilayer film which combines an especially high degree of tamper-resistance with transparency as well as a simple, relatively inexpensive method of manufacture and none has fulfilled the requirements laid out for protecting the data on documents and labels.
One object of the invention is thus to provide a new multilayer film, intended especially for the protection of data on documents as well as for tamper-proof labels, which shows a high degree of tamper-resistance, that is, where tampering can be detected even after alterations have been made by a professional tamperer, and which in addition is simple and inexpensive to manufacture.